This invention relates to light sources for providing stable wavelength light for optical communication systems.
Optical fiber communication systems are beginning to achieve their great potential for the rapid transmission of vast amounts of information. In essence, an optical fiber system comprises a light source, a modulator for impressing information on the light, an optical fiber transmission line for carrying the optical signals and a receiver for detecting the signals and demodulating the information they carry. The optical signals are typically within a wavelength range favorable for propagation within silica fibers and increasingly are wavelength division multiplexed (WDM) signals comprising a plurality of wavelength distinct wavelength channels within that range. Information is typically impressed upon the signal channels as by pulse code modulation.
Wavelength stability is important for the optical sources used in such systems. In contemplated systems, laser sources will be required to maintain their precise operating wavelengths over twenty years of field operation. It is doubtful that even the best current lasers can operate within the tight wavelength tolerances that projected wavelength channel spacings will require. Accordingly, there is a need for light sources with enhanced wavelength stability.
Mach-Zehnder waveguide interferometers for wavelength measurement have been proposed. See, for example, M. Teshima et al., xe2x80x9cMultiwavelength simultaneous monitoring circuit employing wavelength crossover properties of arrayed waveguide gratingxe2x80x9d, Electronics Letters, 31, pp. 1595-97 (1995). The difficulty with such devices, however, is that they are relatively inefficient, bulky, and not readily incorporated into a light source.
In accordance with the invention, a wavelength stable optical source comprises at least one adjustable wavelength optical source, a multipath Mach-Zehnder interferometer (MMZI) for receiving a signal from the adjustable source and providing a primary output and one or more secondary outputs, and a feedback arrangement responsive to the outputs for adjusting the wavelength source. Photodetectors coupled to the primary output and one or more of the secondary outputs provide feedback information for maintaining wavelength stability.